"Phenolic resin" is a term that describes a wide variety of resin products which result from the reaction product of phenols with aldehydes. Phenols react with formaldehydes under both acidic and basic conditions. If a base catalyzed mixture of phenol and formaldehyde contains one or more moles of formaldehyde per mole of phenol, it will produce a thermosetting (one-step) resin. Such resins which are termed "resoles" are commonly used commercially. Common base compounds which are employed as a catalyst for resole resins include the hydroxides of alkali metals such as sodium, potassium, or lithium. While alkali metal hydroxide catalyzed phenolic resins have been in use for a considerable period of time, they have one significant disadvantage. Such resins darken as they age, are heated or are otherwise cured. (See e.g., Chemistry and Application of Phenolic Resins, A. Knop and W. Scheib, Springer-Verlg, 1979, p. 185). The darkening is believed to be dependent on the presence of negative ions from the metal hydroxide catalyst causing loss of hydrogen in ionic form from the phenolic hydroxyl groups, a condition which favors oxidation, resulting in the formation of the intensely red phenoquinone. (See, e.g., Chemistry of Organic Compounds, C. Noller, W. B. Saunders Co., Philadelphia and London, 1965, p. 553 and "The Effect of Catalysts on the Thermal Stability of Phenolic Resins", D. B. V. Parker, Royal Aircraft Establishment Technical Note No. CHEM. 1229, March, 1957, p. 6) The extent of darkening is known to be dependent on the curing or use temperature of the resin and the time of exposure to such temperature.
Alkali metal hydroxide catalyzed phenolic resins are commonly used as a component of the bond system of abrasive products, such as coated, bonded, and three-dimensional, low density abrasive products. The resin darkening problem is particularly pronounced in coated abrasive and three-dimensional, low density abrasive products because of the more visible presence of the bond system. Furthermore, abrasive bond systems may comprise colorants to identify the manufacturer, type of product, application, etc. The darkening of the resin can interfere with the desired coloration, resulting in an unwanted variation in the product color. Since the darkening increases with the temperature and exposure time, the product color may be variable, and thus unacceptable for aesthetic or other reasons.
Page 7 of Parker discloses cured resoles, also designated as "resols", catalyzed by ammonia, magnesia, or small quantities of sodium hydroxide are yellow-brown or orange rather than the purple or nearly black color produced when larger quantities of sodium hydroxide are used. The focus of Parker's work, however, was on the effect of catalysts on the thermal stability of phenolic resins.
Resol comprising ammonium-based salts have been used in articles unrelated to abrasive products. For example, U.S.S.R. Pat. No. 526521, published Aug. 30, 1976, discloses a pressed article having a light tone, wherein the article contains wood filler, resol, fire proofing agents comprising diammonium hydrogen phosphate and ammonium sulfate, a hardener, and a lubricant.
Japanese Pat. No. 49113895, published Oct. 30, 1974, teaches a transparent cured resol, prepared by refluxing sodium hydroxide catalyzed phenolic resin and ammonium chloride and then removing any particulate by filtration. The abstract of this Japanese patent does not disclose a use for the transparent resin, nor does it suggest a color stabilized resol.
Japanese Pat. No. 56027392, published Mar. 17, 1981, describes the use of acid ammonium salts as additives to a resol to provide improved storage stability for color-developer sheets.
The use of ammonium-based salts as grinding aids in grinding wheel type abrasive articles is known in the art. For example, U.S. Pat. No. 2,308,983 discloses abrasive articles, such as grinding wheels, containing a fluoroborate, such as ammonium fluoroborate or an alkali metal fluoroborate (e.g. potassium fluoroborate or sodium fluoroborate) with improved performance characteristics.
U.S. Pat. No. 2,952,529 teaches sulfur-free resinoid bonded abrasive wheels comprising ammonium chloride and cryolite which offer stainless steel cut performance approximately equal to wheels containing sulfur or sulfide fillers. A sulfur-free resinoid bonded abrasive wheel containing cryolite, ammonium chloride, and chilled iron grit to provide improved heat resistance is disclosed in U.S. Pat. No. 2,949,351.
A grinding wheel comprising ammonium chloroferrate or ammonium chlorofluoroferrite as a grinding aid is disclosed in U.S. Pat. No. 4,263,016, and 4,370,148, respectively.
U.S. Pat. No. 4,500,325 discloses an abrasive article in the form of an abrasive disk comprising
A.sub.x Me.sub.y.sup.II Me.sub.z.sup.III Hal.sub.E.nB.sub.f C.sub.g Hal.sub.e.mH.sub.2 O.oNH.sub.3, wherein A is an alkali metal ion or ammonium ion; x is a number between 0 and 10; Me.sup.II is a bivalent metal ion, i.e. Mn, Ca, Mg, Zn, Sn, Cu, Co, or Ni; y is a number between 0 and 2; Me.sup.III is a trivalent metal ion, i.e. Al, B, Ti, z is a number between 0 and 2; Hal represents a halogen; E is a number between 1 and 10; n is a number between 0 and 10; B is an alkali metal ion or ammonium; f is a number between 0 and 1; C represents bivalent element (e.g. Ca, Mg, Zn, Sn, or Mn); g is a number between 0 and 1; e is a number between 1 and 2; m is a number between 0 and 10; and o is a number between 0 and 10.
U.S. Pat. No. 4,877,420 teaches abrasive bodies such as grinding wheels or cutting wheels having halogen-containing compounds as a filler and including ammonium-halogen-based compounds.
Grinding wheels comprising anhydrides of strong inorganic acids or acid salts of strong inorganic acids with ammonium are described in U.S. Pat. No. 2,243,049.
U.S. Pat. No. 4,381,188 discloses an abrasive disk comprising abrasive grains, a bonding agent and pellets, wherein the pellets further comprise a binding agent (including phenolic resin), a pulverulent filler, and ammonium chloride.
Coated abrasive and three-dimensional, low density coated abrasive articles differ significantly from bonded abrasive articles such as grinding wheels or cutting wheels. For example, grinding wheels are typically formed as a relatively deep or thick (three-dimensional) structure of abrasive granules adhesively retained together in a wheel. In contrast, a coated abrasive article typically comprises a support member, abrasive granules, and one or more layers of a bond system which serve to bond the abrasive granules to the support member. A coated abrasive article may further comprise additional non-bonding layers such as, for example, a supersize. Furthermore, a coated abrasive article generally has a significantly higher ratio of bond system to abrasive granules than a grinding wheel.
A three-dimensional, low density abrasive article comprises a three-dimensional, low density web structure, abrasive granules, and a bond system which serves to bond the abrasive articles to the web structure. Like a coated abrasive, a three-dimensional, low density abrasive article generally has a significantly higher ratio of bond system to abrasive granules than a grinding wheel. Furthermore, a three-dimensional, low density abrasive article typically has a void volume within the range from about 85% to 95% whereas the void volume of a grinding wheel is usually substantially less than 85%.
Assignee acknowledges that coated abrasive product comprising alkali metal hydroxide catalyzed phenolic resin and ammonium fluoroborate was sold in the 1970's. The ammonium fluoroborate was an occasional contaminate in a potassium fluoroborate grinding aid which was present in the size or supersize layer of some coated abrasive products. Although alkali metal hydroxide catalyzed phenolic resin comprising the ammonium fluoroborate-contaminated potassium fluoroborate was observed to affect the color of the resin, the effect was viewed as undesirable. The ability to color stabilize a cured resole with ammonium fluoroborate or any other ammonium-based salt was not recognized.
The art does not disclose or teach a means of providing coated or three-dimensional, low density abrasive articles comprising color stabilized alkali metal hydroxide catalyzed phenolic resin.